“They have not been in touch or communicated about this work of theirs to anyone in the curl project.”
Microsoft ships xCurl as a Game Development Kit version of the libcurl API. It is a stripped-down build based on libcurl 7.69 with no DoH, no proxy support and no cookies. Its documentation is a link to curl’s documentation. Microsoft borrowed the API, the docs and the license file, and skipped the email.